The present invention relates to a block copy sheet for lithographic printing plate for preventing the positional deviation of the lithographic printing plate on a plate cylinder of a printing machine.
In a lithographic printing machine, a lithographic printing plate is generally wound on and mechanically fixed to a plate cylinder to effect printing.
Heretofore, as the support for lithographic printing plate there has been used one made of metal, plastic film, paper or the like. A lithographic printing plate comprising a support made of a material other than metal exhibits an excellent handleability but an insufficient dimensional stability as compared with one comprising a support made of a metal.
Such a lithographic printing plate comprising a support made of a material other than metal at least on the back surface thereof, if mounted on a lithographic printing machine, tends to be poorly positioned when caught by the forward end of the plate cylinder because the support is soft. As a result, the precision in longitudinal positioning (precision in positioning in the direction along the circumference of the plate cylinder) is deteriorated. In some cases, the lithographic printing plate may be fixed obliquely deviated. Furthermore, there is a problem that the lithographic printing plate shows a deformation at some areas due to friction with the plate cylinder during printing to thereby cause deterioration of precision in positioning with respect to printing paper.
Accordingly, such a lithographic printing plate comprising a support made of a material other than metal at least on the back surface thereof is used only for printing of so small a number of sheets that no significant problems occur even if the precision in registration on printed matter is low. When used in multi-color precision printing or in printing of a large number of sheets using a large-sized printing machine, misregistration of colors may be caused.
On the other hand, plate-making method and printing method by CTP (computer to plate), which has recently spread, are advantageous in that the precision in dimension and positioning of image (exposure) with respect to the plate material, making registration in multi-color printing easy, as compared with the conventional plate making method and printing method (by contact exposure of plate material using a lithographic film).
However, due to the foregoing problems, the lithographic printing plate comprising a support made of a material other than metal such as plastic film and paper cannot make the best use of the advantage of CTP that registration in multi-color printing is easy.
In recent years, it has been proposed to provide a sheet material having an initial elasticity of 300 kg/mm2 or less interposed between the printing plate and the plate cylinder to solve the foregoing problems (JP-A-11-20130 (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d)). This sheet has fine glass beads adhered thereto so that the central line average roughness Ra is 2 or more.
However, as can be seen in the description of the above cited patent application that the sheet is prepared by densely and uniformly fixing fine glass beads or the like to the surface of a sheet-like material, the formation of roughness on the sheet material requires the use of a high concentration dispersion of fine particles.
In general, this kind of fine particles is expensive. Accordingly, in order to densely and uniformly fix such fine particles to the sheet-like material, a large amount of fine particles is required, adding to the cost of the sheet. Further, it is difficult to disperse fine particles in a liquid in a high concentration. Moreover, when dispersed in a high concentration, such fine particles can agglomerate in the liquid to form coarse particles that deteriorate the print quality.
It is therefore an object of the present invention to provide a block copy sheet for lithographic printing plate for preventing the positional deviation of a lithographic printing plate which can securely prevent the positional deviation of a lithographic printing plate on the plate cylinder of a press, making it possible to apply a lithographic printing plate comprising a support made of a material other than metal also to multi-color printing or printing of a large number of sheets, prevent the positional deviation of the printing plate, reduce the production cost and give an excellent productivity.
Other objects and effects of the present invention will become more apparent from the following description.
The above-described objects of the present invention have been achieved by providing the following block copy sheets.
In a first aspect, the present invention is directed to a block copy sheet for lithographic printing plate to be interposed between a plate cylinder and a lithographic printing plate having a front surface and a back surface, at least the back surface being made of a material other than metals,
wherein said block copy sheet have convex portions provided on a surface thereof, said convex portions having a predetermined shape so as to concave the back surface of the lithographic printing plate when pressed against the back surface of the lithographic printing plate, and
wherein said convex portions comprises a composite particle having protrusions provided on the surface thereof and having a hardness greater than that of the back surface of the lithographic printing plate and a particle diameter of from 1 xcexcm to 100 xcexcm.
The composite particle may comprise an inorganic material, an organic material, or an organic-inorganic composite material.
The sum of the maximum sectional area of said convex portions parallel to the surface of the sheet per unit area is preferably from 0.05% to 4%.
In a second aspect, the present invention is directed to a block copy sheet for lithographic printing plate to be interposed between a plate cylinder and a lithographic printing plate having a front surface and a back surface, at least the back surface being made of a material other than metals,
wherein said block copy sheet have convex portions provided on a surface thereof, said convex portions having a predetermined shape so as to concave the back surface of the lithographic printing plate when pressed against the back surface of the lithographic printing plate,
wherein the contact area of said convex portions and the back surface of the lithographic printing plate is from 0.6% to 3.0% of the area of the back surface of the lithographic printing plate, and
wherein said convex portions comprises non-spherical fillers having an average length of from 1 xcexcm to 200 xcexcm.
The non-spherical fillers may be tabular-shaped, acicular-shaped or tetrapod-shaped.